PURPOSE: To stabilize the control of an apparatus and to reduce the cost of said control by generating a voltage waveform similar to the collector voltage waveform of a switching transistor in the detection winding of a converter transformer and by signal-processing said voltage waveform to generate a synchronizing signal.

CONSTITUTION: A switching power supply is composed of a converter transformer 1, a switching transistor(Tr) 2, a damper diode 3, a resonance capacitor 4, a pulse width control circuit 6, a photocoupler 7, and an error detection circuit 8. Also, the synchronous generator circuit of said power supply is equipped with a level regulator circuit 25, a rectifying and smoothing circuit 26, a phase compensating circuit 27, and a comparator circuit 28, and applies a collector output signal of Tr 21 of said comparator circuit 28 as synchronizing signal V3 to said control circuit 6 to control the ON-OFF state of said Tr 2. Thus, the voltage of a detection winding is rectified and smoothed into a threshold value also with regard to the variation of input voltage so as to enable obtaining said threshold voltage with a magnitude proportional to that of input voltage.